In recent years, different types of cellular networks for wireless communication have been developed to provide radio access for various wireless terminals in different areas. The cellular networks are constantly improved to provide better coverage and capacity to meet the demands from subscribers using services and increasingly advanced terminals, e.g. smartphones and tablets, which often require considerable amounts of bandwidth and resources for data transport in the networks. As a result, it is common to configure a network with cells of varying types and sizes, e.g. in an overlapping fashion, to provide needed capacity and flexibility depending on expected traffic intensity in different areas, the cells forming a so-called heterogeneous cellular network.
In this disclosure, the term “User Equipment, UE” will be used to represent any user-controlled wireless terminal or device capable of radio communication including receiving downlink signals transmitted from a radio node of a wireless communication network. Further, the term “radio node”, also commonly referred to as a base station, e-nodeB, eNB, etc., represents any node of a wireless communication network that can communicate uplink and downlink radio signals with UEs. The radio nodes described here may include so-called macro nodes and low power nodes.
A heterogeneous cellular network thus comprises hierarchically arranged radio nodes, e.g. including macro nodes transmitting with relatively high power and covering relatively large areas of a size in the order of kilometers, and low power nodes transmitting with relatively low power and covering areas of a size in the order of a few meters, e.g. micro, pico, femto and relay nodes, to mention some customary examples. The low power nodes may be employed together with the macro nodes in an overlapping fashion to locally provide added capacity in so-called “hot spot” areas such that multiple small areas served by such micro/pico/femto/relay nodes may be located within the area served by a macro node.
The above-described heterogeneous network may be realized such that a macro node and multiple low power nodes cover individual cells with different cell identifiers, which means that a UE is served by one radio node at a time and must undergo handover between the cells when necessary to maintain adequate radio coverage. Alternatively, the macro node and the low power nodes may instead cover the same common cell with a single cell identifier, which means that a UE in the cell can be connected to and served by multiple radio nodes at the same time. The radio nodes of such a combined cell, sometimes also called a “soft cell” or “shared cell”, can be regarded as a distributed radio node with multiple antennas at separate physical locations in the cell. In this disclosure, the term combined cell is used to represent such a cell served by multiple radio nodes using the same cell identifier. It should be noted that a combined cell may, without limitation, comprise any number of macro nodes and low power nodes.
The latter alternative of using a combined cell covered by multiple radio nodes has the advantage of eliminating the need for performing handover which reduces the amount of signaling both in the radio interface and in the network, and also avoids the risk of dropped connection due to failed handover, among other things. FIG. 1 illustrates an example of a combined cell being served by a macro node 100 serving the entire cell, more or less, and a set of low power nodes 102-108 serving considerably smaller areas, not shown. Depending on its location, a UE present in the cell may receive downlink signals from any of the radio nodes 100-108 as suggested by dashed arrows.
Uplink radio signals, e.g. containing data or control information, transmitted from the UE may also be received by several, if not all, of the radio nodes which are able to process the radio signals jointly or in some coordinated way. It is thus another advantage with the above-described concept of combined cell that it enables an operation of signal combining on uplink signals from the UE when received by the multiple radio nodes. The operation of signal combining on signals received by two or more physically separated antennas is well-known as such in this field and is not necessary to describe in any detail to understand this disclosure. For example, each node may receive and decode the radio signals from the UE and may further perform channel estimation individually which can be utilized in a coordinated manner to achieve correct decoding.
However, It is thus a problem that the operation of signal combining on uplink radio signals transmitted from a UE present in a combined cell may be imperfect and even unsuccessful which naturally degrades the communication with the UE. As a result, the UE may be required to make frequent re-transmissions and/or increase its transmit power, which typically increases the level of interference in the cell as well as in one or more neighboring cells. If the signal combining operation is unsuccessful, the connection may even be lost altogether and the radio communication with the UE is consequently interrupted.